CHONDRULE FORMATION IN LIGHTNING DISCHARGES

Chondrules represent a significant mass fraction of primitive meteorit es. These millimeter-sized glassy droplets appear to be the products o f intensive transient heating events. Their size distribution, chemica l and mineral composition, texture, and isotope composition suggest th at chondrules were produced as a result of short duration melting foll owed by rapid cooling of solid precursor particles. Gas dynamic heatin g, magnetic reconnection, and electrostatic discharges are thought to be the leading candidates to explain chondrule formation. In this pape r we model the effects of ''lightning'' in the early Solar System. Dif ferential settling of various sized dust particles toward the midplane of the nebula is suspected to build large scale charge separations th at episodically relax via the electric breakdown of the nebular gas. T he electrostatic discharge is analogous to lightning in the Earth's at mosphere. In this paper we do not discuss the complex physical process es that may lead to electric field generation. Instead, assuming that this may occur due to a number of inductive and noninductive processes , we use the expected initial conditions in such a discharge. We then follow the expansion of the initially energetic plasma column as it ex pands, cools, and recombines. We calculate the energy flux reaching th e surface of an embedded dust grain and also its subsequent heating. W e show that, within the range of expected initial plasma conditions, l ightning is a viable mechanism for chondrule formation worthy of more complex theoretical and also laboratory investigations. (C) 1995 Acade mic Press, Inc.